SUMMARY OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetostriction oscillator which is used for generating a high output ultrasonic wave, more particularly, to a circuit and a method for driving a magnetostriction oscillator by using a pulse width modulation.
2. Prior Art
Generally, a method which removes or prevents scales in heat exchange equipment is performed by washing a pipe by means of a strong acid, or by using an ultrasonic wave generated by means of a magnetostriction oscillator.
An impulse voltage of a high voltage is alternately supplied to each of a pair of magnetostriction oscillators. This removes scales or prevents a generation thereof by an ultrasonic vibration by emitting an ultrasonic wave generated by each magnetostriction oscillator to an inner wall of a pipe in the heat exchanger equipment.
U.S. Pat. No. 3,633,424 issued to Lawrence W. Lynnworth et al. on Jan. 11, 1972 discloses a magnetostriction ultrasonic transducer in which the number of unwanted pulses due to internal reflections is minimized.
FIG. 1 is a circuitry diagram for showing a conventional magnetostriction oscillator driver.
The conventional magnetostriction oscillator driver includes first and second diodes D1 and D2, first and second capacitors C1 and C2, and first and second silicon controlled rectifiers SCR1 and SCR2. When an alternating current voltage AC is applied to first and second diodes D1 and D2, the first and second diodes D1 and D2 perform half-wave rectification of the AC for half cycle by forward and reverse directions. The first and second silicon controlled rectifiers SCR1 and SCR2 alternately perform switching operations so that a predetermined distributed voltage is supplied to first and second magnetostriction oscillators M1 and M2. A resistor R3, a zener diode D3, and a diode D5 are connected to a gate of the first silicon controlled rectifier SCR1 in series. The resistor R3, zener diode D3, and diode D5 serve to switch the first silicon controlled rectifier SCR1. A resistor R4, a zener diode D4, and a diode D6 are connected to a gate of the second silicon controlled rectifier SCR2 in series. The resistor R3, zener diode D3, and diode D5 serve to switch the second silicon controlled rectifier SCR2.
A resistor R5 and a capacitor C4 are connected to an anode of the diode D5 and generate a switching signal according to a time constant thereof. A resistor R6 and a capacitor C5 are connected to an anode of the diode D6 and generate a switching signal according to a time constant thereof. A capacitor C3 is connected between the first and second silicon controlled rectifiers SCR1 and SCR2 and serves to form the predetermined distributed voltage. Resistors R1 and R2 are connected to the capacitors C1 and C2 in parallel, respectively.
An operation of the conventional magnetostriction oscillator driver will now be described.
When an alternating current voltage AC is applied to the first diode D1 through a fuse F for half cycle by a forward direction, the first diode D1 performs half-wave rectification of the AC and outputs a half-wave rectified voltage. The half-wave rectified voltage from the first diode D1 is smoothed by a capacitor C1 and a resistor R1, and the smoothed voltage is supplied to a resistor R5 and a capacitor C4 through a first magnetostriction oscillator M1 to thereby charge the capacitor C4.
When a predetermined time according to a time constant formed by the resistor R5 and capacitor C4 elapses, the capacitor C4 discharges the charged voltage through a path formed between the capacitor C4 and a diode D5. When the discharge voltage is higher than a predetermined voltage, the zener diode D3 triggers a gate of the first silicon controlled rectifiers SCR1 through a resistor R3. Accordingly, the smoothed voltage from the capacitor C1 defines a closed circuit through a capacitor C3 by the switching of the first silicon controlled rectifiers SCR1. At this time, since the capacitor C1 and C3 are connected in parallel to each other, excessive current flows through the first magnetostriction oscillator M1 by the parallel capacitances of the capacitors C1 and C3. Accordingly, an impulse of a high voltage is generated in the first magnetostriction oscillator Ml so that the first magnetostriction oscillator Ml oscillates an ultrasonic signal.
On the other hand, when the alternating current voltage AC is applied to the second diode D2 through a fuse F for half cycle by a reverse direction, the second diode D2 performs half-wave rectification of the AC and outputs a half-wave rectified voltage. The half-wave rectified voltage from the second diode D2 is smoothed by a capacitor C2 and a resistor R2, and the smoothed voltage is supplied to a resistor R6 and a capacitor C5 through the charged capacitor C3 to thereby charge the capacitor C5.
When a predetermined time according to a time constant formed by the resistor R5 and capacitor C4 elapses, the capacitor C5 discharges the charged voltage through a path formed between the capacitor C5 and a diode D6. When the discharge voltage is higher than a predetermined voltage, the zener diode D4 triggers a gate of the second silicon controlled rectifiers SCR2 through a resistor R4. Accordingly, the voltage charged in capacitors C2 and C3 is discharged into the second magnetostriction oscillator M2. The discharged voltage, that is, an impulse of a high voltage operates the second magnetostriction oscillator M2.
As described previously, when a high voltage is alternately applied to first and second magnetostriction oscillators M1 and M2 according to half cycles by forward and reverse directions of the AC power, first and second magnetostriction oscillators M1 and M2 vibrate to generate an ultrasonic signal of a predetermined frequency. Such an ultrasonic signal is used for removing scales or preventing generation thereof.
In the conventional magnetostriction oscillator driver, a high voltage of 1000 volts is used. This means that output efficiency of an ultrasonic wave is lower than input power so that power for operating a system is wasted. An increase of noise due to non-resonant vibrations of magnetostriction oscillator and peripheral circuits thereof according to an impulse of high voltage causes fatigue fracture so that the system cannot be used for a long time. In order to operate the system, two magnetostriction oscillators should be alternately used. Thus, when something is wrong with either one of the two, the system cannot operate.